Apparatus for controlling a piston disposed in a cylinder closed at each end is already known, this cylinder being controlled by a valve with two outlet orifices, adapted to connect at least one of the spaces located on each side of the piston alternatively to a source of fluid under pressure and an exhaust conduit.
In the case of a double-acting cylinder comprising a piston displaceable in a chamber closed at each end, the piston is controlled by a four-passage valve, with two outlet orifices, adapted to connect alternatively the spaces located on each side of the piston to a source of fluid under pressure and to one or two exhaust conduits which return the fluid to a reservoir in the case of oil.
In other embodiments, the cylinder comprises only one space to which is fed fluid under pressure. In this case, it is a single-acting cylinder.
It is known that the two types of cylinders may be controlled by the same type of valve by stopping the unused openings in the case of the control of a single-acting cylinder.
In this last case, the space opposite that provided to be placed under pressure communicates with the atmosphere and, under the effect of the reciprocation of the piston, draws in and then ejects ambient air. The piston being generally lubricated--even if the fluid under pressure is compressed air--the lubricant is therefore in contact with the ambient air, which is unacceptable in certain types of application, particularly in food, pharmaceutical, chemical industries where it is absolutely necessary to avoid contamination of the products by lubricant, even in infinitesimal quantities. To overcome this drawback, said space should be closed and be connected to a conduit permitting recovery of the air and the oil vapors, which requires a special installation.